1. Field of the Invention
The present application relates to variable focal length lenses and in particular to lens systems for use with micrographic projectors and printers.
2. Description Relative to the Prior Art
In order to conserve space, documents are often stored photographically with their size greatly reduced on microfilm, microfiche, or other media. Such processes and media are known generally by the term micrographics.
In order to utilize a document which has been stored in micrographic format, a projector or reader is required. Such a reader will produce an enlarged reproduction of the reduced image on a screen for viewing by the operator. Often such readers also include a printer. When a desired document has been located by projection onto the screen, the operator may produce a permanent enlarged copy through photographic or plain paper copier techniques. Systems which permit such reading and printing are known as reader/printers. The term reader/printer will be used herein to denote both readers containing only a projector and those including a printer in addition to a projector.
A problem which arises in the use of such systems relates to the fact that different reduction factors may be used in recording the micrographic images and that recorded documents may be of different sizes. As a result, different magnifications are required in order to reproduce properly such images or to make a reproduced image fill a desired area. In order to accommodate such variations, micrographic reader/printers are commonly provided with lenses having a variety of magnifications. In some cases, as many as fifteen or more lenses are required to accommodate all possible enlargement factors that may be desired. The frequent changing of lenses can create a great inconvenience to operators of such equipment and can increase the time required for reading and printing documents stored in micrographic form.
U.S. Pat. Nos. 4,750,820; 4,743,102; 4,746,204 and 4,735,951 each disclose a zoom lens as a solution to the above problem. Because each zoom lens provides a variety of magnifications, at most only a few zoom lenses are needed to cover the entire desired magnification range. Zoom lenses disclosed in U.S. Pat. Nos. 4,750,820; 4,743,102 and 4,733,951 have an aperture stop located between the two zooming groups. This type of design usually causes as much as 50% or more vignetting of the light in the corners of the field relative to the axis. With such gross amounts of vignetting, depending upon the angular coverage, the relative illumination of such optical systems can fall to 30%. This is very typical of camera systems. When this occurs in microfilm systems, the observer is forced to stop down the lens system to eliminate the annoying visual effect. However, when this is done the overall illumination dictated objectionable exposure durations. In a reader/printer a zoom lens is commonly located behind a collimator and Dove prism rotator assembly, which is used to rotate the image, and it is obviously important to minimize the size of this assembly. Accordingly, the lens elements near the rotator assembly should be kept small, which can be accomplished by locating the aperture stop near that assembly. U.S. Pat. No. 4,733,951 has an aperture stop located in front of the two zooming optical units. Since the two optical units are a front positive optical unit and a rear negative optical unit, this is a telephoto-type lens. A telephoto lens generally results in a short back focal length compared to its effective focal length. Because in telephoto-type lenses back focus gets smaller with higher magnification, the back focus can impose an upper limit on the obtainable magnification. In addition, having film immediately adjacent to lens elements is inconvenient because of supporting difficulties and, because, if located next to the focal plane, the lens elements themselves need to have good scratch and dig tolerances because scratches and digs in the lens surface will be imaged on the focal plane. For the same reason, if dust or dirt gets in the back of such a system, this will be seen in the image; which require these lens elements to be kept very clean.
U.S. Pat. No. 3,724,927 also discloses a zoom lens system designed for micrographic applications. However, this is a complicated system involving three separate lens assemblies and requires a very long lens with many lens elements, which is inconsistent with the desirability for compactness.
The present invention overcomes the foregoing problems by means of a reader printer zoom lens for use in conjunction with an image rotator assembly to provide a wide range of magnification; minimum vignetting and constant, high illumination; a constant object-to-image distance; a half field coverage greater than 10 degrees; and advantageous utilization of a non-collimated light entering an aperture stop. Also, the optical elements located near the rotator assembly are of relative small aperture and the size of the optical components of that assembly is also correspondingly small.
Other advantages and novel features will become apparent from the following description of illustrative embodiments of the invention illustrated in the accompanying drawings.